Mysterious Cosmic 'Dark Flow' Tracked Deeper into Universe (w/ Video)

March 10, 2010

The colored dots are clusters within one of four distance ranges, with redder colors indicating greater distance. Colored ellipses show the direction of bulk motion for the clusters of the corresponding color. Images of representative galaxy clusters in each distance slice are also shown. Credit: NASA/Goddard/A. Kashlinsky, et al.

(PhysOrg.com) -- Distant galaxy clusters mysteriously stream at a million miles per hour along a path roughly centered on the southern constellations Centaurus and Hydra. A new study led by Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md., tracks this collective motion -- dubbed the "dark flow" -- to twice the distance originally reported.

"This is not something we set out to find, but we cannot make it go away," Kashlinsky said. "Now we see that it persists to much greater distances -- as far as 2.5 billion light-years away." The new study appears in the March 20 issue of The Astrophysical Journal Letters.

The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain. Evidence indicates that the clusters are headed outward along this path, away from Earth, but the team cannot yet rule out the opposite flow. "We detect motion along this axis, but right now our data cannot state as strongly as we'd like whether the clusters are coming or going," Kashlinsky said.

The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe -- outside our "horizon" -- is pulling on matter in our vicinity.

Cosmologists regard the microwave background -- a flash of light emitted 380,000 years after the universe formed -- as the ultimate cosmic reference frame. Relative to it, all large-scale motion should show no preferred direction.

The hot X-ray-emitting gas within a galaxy cluster scatters photons from the cosmic microwave background (CMB). Because galaxy clusters don't precisely follow the expansion of space, the wavelengths of scattered photons change in a way that reflects each cluster's individual motion.

This results in a minute shift of the microwave background's temperature in the cluster's direction. The change, which astronomers call the kinematic Sunyaev-Zel'dovich (KSZ) effect, is so small that it has never been observed in a single galaxy cluster.

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But in 2000, Kashlinsky, working with Fernando Atrio-Barandela at the University of Salamanca, Spain, demonstrated that it was possible to tease the subtle signal out of the measurement noise by studying large numbers of clusters.

In 2008, armed with a catalog of 700 clusters assembled by Harald Ebeling at the University of Hawaii and Dale Kocevski, now at the University of California, Santa Cruz, the researchers applied the technique to the three-year WMAP data release. That's when the mystery motion first came to light.

The new study builds on the previous one by using the five-year results from WMAP and by doubling the number of galaxy clusters.

"It takes, on average, about an hour of telescope time to measure the distance to each cluster we work with, not to mention the years required to find these systems in the first place," Ebeling said. "This is a project requiring considerable followthrough."

According to Atrio-Barandela, who has focused on understanding the possible errors in the team's analysis, the new study provides much stronger evidence that the dark flow is real. For example, the brightest clusters at X-ray wavelengths hold the greatest amount of hot gas to distort CMB photons. "When processed, these same clusters also display the strongest KSZ signature -- unlikely if the dark flow were merely a statistical fluke," he said.

In addition, the team, which now also includes Alastair Edge at the University of Durham, England, sorted the cluster catalog into four "slices" representing different distance ranges. They then examined the preferred flow direction for the clusters within each slice. While the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement.

The researchers are currently working to expand their cluster catalog in order to track the dark flow to about twice the current distance. Improved modeling of hot gas within the galaxy clusters will help refine the speed, axis, and direction of motion.

Future plans call for testing the findings against newer data released from the WMAP project and the European Space Agency's Planck mission, which is also currently mapping the microwave background.

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The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe -- outside our "horizon" -- is pulling on matter in our vicinity.

From a General Relativity point of view, this makes no sense. Gravity travels at the speed of light, so no gravitational effect from "outside our 'horizon'" can possibly have reached us yet, by the very definition of the light horizon. If we can feel the gravity from that mysterious matter, then we ought to be seeing the light from it as well, and it can't be any farther out than 13.7 billion light-years.

Either that, or the author(s) of the article contend that they've disproven the theory of General Relativity...

Either that, or the author(s) of the article contend that they've disproven the theory of General Relativity...

But of course. There is a string theorist in the US (I forget her name. I am horrible with names,) who wrote a paper completely routed in M-theory, and since it is considered "The only working theory on gravity" as per the popular news rags, it makes sense that Relativity was thrown out of the door.

The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe -- outside our "horizon" -- is pulling on matter in our vicinity.

Well, it would help explain why everything seems to be accelerating away from us (and everything else).

From a General Relativity point of view, this makes no sense. Gravity travels at the speed of light, so no gravitational effect from "outside our 'horizon'" can possibly have reached us yet, by the very definition of the light horizon. If we can feel the gravity from that mysterious matter, then we ought to be seeing the light from it as well, and it can't be any farther out than 13.7 billion light-years.

Either that, or the author(s) of the article contend that they've disproven the theory of General Relativity...

For all that you wrote to be true, you would need:

• the Big Bang theory to be absolutely true• the 13.7 Bly radius of the Universe to be correct• the 2*13.7 billion years to be refferenced as the "start of time"• that the Universe is only as big as far we can currently see

Although many currently famous theorists would agree with all the above as being true, I for myself, beg to differ.

This dark flow motion together with cosmological electromagnetic anisotropy together seem to point to some sort of directed universal evolution of even supposed inorganic matter that no sanitized purely mathematical model can account for. It is evidence that no matter how long one attempts to achieve a so-called unified field theory the universe will always present us with some unexpected singularities due to its continuing evolution which no purely imaginary mathematical can ever fully explain.

FYI: to the above posters Dark flow and Dark matter are not necessarily linked. The use of the term Dark means either we can't see it or it is indeterminant, ie: dark within our understanding, not necessarily by nature.

One interesting thought I've entertained was the Big Crunch. Perhaps our observational frame of reference is wrong. If we were closer to the edge of reality, or effectively an elder statesman amongst galaxies, we could rather easily be misintepreting the "expansion" of the Universe and actually be on the lagging end of a compression mechanism.

In any event our increasing ability to produce deeper and deeper scans on the universe should prove rather enlightening once we can "view" out to a range where this supposed attractor is.

PinkElephant: your statement that "gravity travels at the speed of light" is an assumption, purely theoretical. In fact, in my model of the Universe, gravity occurs as the interaction of objects in a dynamic web, or field, in which changes occur instantaneously, like entangled photons. That is why the effort to detect gravitational waves is an exercise in futility, likewise searching for the graviton. Experience over time will prove me right, just wait and see.

Most astrophysicists acknowledge that space is every bit as much a creation as is time and matter. Matter displaces space, yet the expanding Universe theory must account for the mutable nature of an expanding space in which the dynamic of the gravity field is constant and relative.

the Big Bang theory to be absolutely truethe 13.7 Bly radius of the Universe to be correct

Both are quite well established at this point. Everything from astrometrics (the light horizon, redshift), to elemental abundance derived from spectra (starting out with mostly hydrogen/helium, then heavy element generation in stars over time), to clustering and structuring of galaxies, etc.

the 2*13.7 billion years to be refferenced as the "start of time"

There's no "2*". 13.7 billion years ago (maybe +/- a billion) is "start of time", at least as far as anything measurable is concerned. Evidence of anything that might have existed prior to that moment, would be pretty well erased by what followed...

that the Universe is only as big as far we can currently see

No, the Universe is infinite as far as anyone knows. We can only see out to 13.7 billion light-years, because light from farther-out hasn't reached us yet.

A 13.7Bly radius would assume that the Universe was not infinite if the Big Bang is correct.

It's really not that complicated: if the universe is 13.7 billion years old, then light can only have traveled a maximum distance of 13.7 billion light-years in a straight line since the Big Bang. Thus, from any given vantage point in space (such as from Earth), you can only see objects up to 13.7 billion light-years away. Light from any objects farther away than that, simply has not yet reached us. As more time passes, we'll be able to see farther and farther out -- as our light horizon continues to expand by one light-year per year in every direction.

Of course, being able to see even 13.7 billion light years out, is rather difficult at this time. It would take a very large telescope of exquisite sensitivity, to pick out anything smaller than a galaxy cluster at such distances...

Yes I understand that, but, with an expanding Universe and your capacity for logic you know that the Universe must be larger than what we can see.

Reasoning for this is simple. Light CANNOT travel in a straight line in the early universe unless gravity doesn't exist. The pull of so much matter in distinct space would immediately delay and redirect said light until the true measurement was obscured.

The easiest way to explain this is a photon of light escaping from the sun. It can take several million years for that photon to escape simply due to being redirected, bent, absorbed and re-emitted billions and perhaps trillions of times. Imagine what the path would look like in the early universe where for millions of years the density of the matter and energy in the universe was far greater than the density of the sun.

The mechanics here are mind boggling. To get even a reasonable approximation of 13.75 Bly is intellectually tenuous at best. Now toss in hyperinflation...

What the 13.75Bly measurement is good for is determining the AGE, not the size of the Universe.

If we cannot see anything older than 13.75Bly away then that means that nothing was emitting light 13.75 Billion years ago, (even this must wrong due to uncalculated hyperinflation and special relativity).

Does that make my assertion a little more clear? I know these things can get supremely confusing and sometimes simplifying only obscures the point even more.

What the 13.75Bly measurement is good for is determining the AGE, not the size of the Universe.

Again, regarding size we can only assume infinity at this point. It's the simplest assumption.

If we cannot see anything older than 13.75Bly away then that means that nothing was emitting light 13.75 Billion years ago

For a few hundred thousand years after Big Bang, the universe was filled with ionized nuclei, and thus opaque to light -- the so-called Dark Ages. Only once electrons recombined with nuclei, was radiation able to propagate freely over long distances. That's the origin of cosmic microwave background.

It then took some more time (dozens or perhaps hundreds of millions of years) for the first stars to begin forming, then quasars, then sizable galaxies. By then, the currently visible volume of the universe was already pretty large, and continuing to expand rapidly. Today, we can only see anything approximately to the edges of this volume.

the Big Bang theory to be absolutely truethe 13.7 Bly radius of the Universe to be correct

Both are quite well established at this point. Everything from astrometrics (the light horizon,redshift), to elemental abundance derived from spectra (starting out with mostly hydrogen/helium, then heavy element generation in stars over time), to clustering and structuring of galaxies, etc.

I was reffering to THE Big Bang theory, which is quite specific on its postulates. Nowhere did I deny the possibility of a bigbang-like event to have happened at some point. In 3000 B.C., it was quite well "established", that the Earth is flat.. ;-)

And yes, you are mostly right about the "2*", though the part where I used it was certainly not about numbers, but I think you got the point..

No, the Universe is infinite as far as anyone knows. We can only see out to 13.7 BLy, because light from farther-out hasn't reached us yet.

As Skeptic allready pointed out, you have some contradictions there. If the Universe was infinite, there would be no "start of time", and so the light would allready had infinite time to reach us. The same applies to the case of gravity mentioned in one of your previous posts.

I don't see the link between putative spatial extent, vs. "start of time". Besides, I'm not one to posit that something can come from nothing. IMHO, the Big Bang was an event within a pre-existing context (and thus, within a pre-existing time.)

...and so the light would allready had infinite time to reach us

As I mentioned above, for quite some time following Big Bang, the universe was opaque to light. Then once it became transparent, there weren't yet any light-emitting objects that we could observe. Those took some more time to form. Only THEN was any light emitted, that we could distinguish from the cosmic microwave background. There's no "infinite time" here to speak of.

The universe can be easily infinite in extent, in which case the Big Bang could have caused simultaneous eruption of matter-energy-space at every point on an infinite continuum (e.g. as in the ekpyrotic hypothesis.)

Oh and by the way, we are also not at the center of the Universe. As far as anyone knows, the Universe has no center...

When we talk of the light horizon, it's just a sphere with a 13.7 billion light-year radius, that happens to be centered on us. Any OTHER point in the universe -- even one at the very edge of our particular light sphere, would have an identical 13.7 billion light-year horizon extending from IT in all directions.

Any OTHER point in the universe -- even one at the very edge of our particular light sphere, would have an identical 13.7 billion light-year horizon extending from IT in all directions.

Unless the Universe actually is a torus. In which case only particular places would have a radius and others would be on a measure of combinant arc length with a similar resultant view. I hope someone understands what I'm talking about, it's very difficult to explain and my english is leaving me at a loss.

It might as well be a pretzel. What's the point of speculating on an infinite range of possibilities? Use Ockham's Razor: the simplest assumption will suffice, unless and until there's empirical evidence against it.

The simplest assumption is that the Universe is infinite in extent, and topologically flat at any fixed time instant (if you could imagine somehow gauging its topology instantaneously everywhere, without regard to causality or light speed.) As far as I know, there's no evidence to contradict such a postulate.

FYI: to the above posters Dark flow and Dark matter are not necessarily linked. The use of the term Dark means either we can't see it or it is indeterminant, ie: dark within our understanding, not necessarily by nature.

This is why you should read stuff. Here's my post again: ------They're not postulating dark matter exactly, but matter outside our observable universe. Which would likely be mostly dark matter (since most matter in the observable universe is dark), but not necessarily. I wonder if this is evidence that ours is a bubble universe that is "off to one side" of the main mass of universes. Or if that even makes sense.-------

Also, you seem to be getting confused between the size of the Universe and the size of the *Observable* Universe.

The time spent in the “dark age” of the Big Bang would have been infinitely long. The early universe was infinitely dense. Time moves slower the closer it is to large mass. Infinite mass would result in infinite amounts of time as perceived by us in our position in space-time today. I believe the correct answer may be that the universe is infinitely old but that we can only see up to the time when the energy condensed enough to be measurable say ~13.7 Billion or so.

The time spent in the "dark age" after the Big Bang would have been infinitely long. The early universe was infinitely dense. Time moves slower at it is closer to higher mass. Infinite mass would result in infinite amounts of time as perceived by us in our position in space-time today.

I believe the correct answer may be that the universe is infinitely old but that we can only measure up to ~13.7 Billion years or so when the mass became dispersed enough to start the clock moving at a fast enough clip.

When we talk of the light horizon, it's just a sphere with a 13.7 billion light-year radius

It doesn't seem to be so simple. Look at http://en.wikiped...etry.pngAn object we see at a distance of 13 billion LY was only 4 billion LY separated from "us" when the light we see now was emitted. And "now" it may be 26 billion LY away. Of course, we can't see it where it is now. We see it where it has been 13 billion LY ago. (The numbers given here are just symbolic, not calculated.)

Also, you seem to be getting confused between the size of the Universe and the size of the *Observable* Universe.

That's a big negative. Although I appreciate your challenge to my statement you've also misread.

Any matter outside of our observable universe cannot be quantified and as such your statement is a mere supposition.

Realistically the "observable" universe and the extent of the Universe are one in the same. It goes back to the perception of the CMB. As we can quantify the CMB and we know that the minor variances within temperature correlate with the presence of abundant mass we can calculate the size and extent of our reality as it evolves based upon our observation.

This predicts that if the universe began to contract we would see an immediate doubling in the intensity of all non-local objects.

You're getting hung up on the fact I'm using 13.7 Bly away as opposed to an assumed potential 40 Bly away.

After reading all that has been written over again + your last posts, I found where the "missunderstanding" came from. Now all gives sense to me and I agree with most of the reasonings. At first it looked to me like you were proposing a classical BB version, with a "start of time", "absolute edge", and all that stuff associated with it - and that is also the reason why I objected.

To me, it looks like the next step in this discussion should be, wether matter that was "just" created (converted) from pure energy, is affected by gravity from distant sources instantly, or with a time-lag equal to the distance divided by the speed of light. With emphasis on the fact, that the energy, from which the matter was made, was allready present BEFORE the conversion took place.

I think it will be the former (instantly)..

Btw PE, that's alot of words just to express that The Bang was a local event instead of "universal" :)

When we talk of the light horizon, it's just a sphere with a 13.7 billion light-year radius

It doesn't seem to be so simple. Look at http://en.wikiped...etry.pngAn object we see at a distance of 13 billion LY was only 4 billion LY separated from "us" when the light we see now was emitted. And "now" it may be 26 billion LY away. Of course, we can't see it where it is now. We see it where it has been 13 billion LY ago. (The numbers given here are just symbolic, not calculated.)

Maybe this movement is toward the so-called "missing mass"-which isn't missing at all- just out of sight- as perhaps it moved "outwards' first at the time of the big bang- a first bubble ejected by initial shock wave- and the light hasn't reached us yet? This is really stretching my ability to conceptualize, given my limited understanding, so my apologies for any murkiness or outright bone-headedness. Does this notion make any sense to anyone?

The time spent in the “dark age” of the Big Bang would have been infinitely long. The early universe was infinitely dense. Time moves slower the closer it is to large mass.

That would be true only from the point of an "external" observer. For example, if you were inside more curved spacetime, you would not notice any difference in the "flow of time". Only an observer that is "outside", or rather "in less/more curved" spacetime, would perceive a difference.

Any matter outside of our observable universe cannot be quantified and as such your statement is a mere supposition.

That's right! Have a cookie. Was it the words "likely" and "not necessarily" in my post that tipped you off? Cuz that's what they were there for.

The article says:

The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe -- outside our "horizon" -- is pulling on matter in our vicinity.

If that's even possible, then no, the Observable Universe and the Universe are not one and the same.

When we talk of the light horizon, it's just a sphere with a 13.7 billion light-year radius

It doesn't seem to be so simple. Look at http://en.wikiped...etry.pngAn object we see at a distance of 13 billion LY was only 4 billion LY separated from "us" when the light we see now was emitted. And "now" it may be 26 billion LY away.

That depends on how you propose to measure distance. In Einsteinian construction, you can only define distance by timing light; outside of light transmission in vacuum, distance has no meaning in GR.

You could say that at time of emission the "real" distance (by what metric?) was 4 billion ly, and at time of arrival the "real" distance (again, by what metric?) is whatever, but in terms of the distance actually covered by the photon: 13 billion ly is the answer.

Admittedly, things would be a lot less muddled in vernacular if spacetime weren't expanding...

BTW, I think the picture you linked contains a conceptual error (or at least, a rather misleading aspect.) The red curve is as it should be, but the orange curve is incorrect: if the quasar emitted a photon from its current position (assuming it's still a quasar, 13 billion years later... haha), that photon would take a spiraling curved path up the expanding "cone", just like the red curve, and consequently would take a lot longer than 26 billion years to reach us (as the universe keeps expanding.)

The orange curve shows an instantaneous distance measurement (ignoring the lightspeed limit), which is physically unsound even if it does have some mathematical interpretation in the context of tensor geometry...

matter that was "just" created (converted) from pure energy, is affected by gravity from distant sources instantly

Not in GR, it isn't. In GR, gravity is fundamentally limited by lightspeed, because the spacetime metric is defined by and derived from light propagation through space. In other words, in GR the lightspeed limit for gravity is tautological: it is so by definition, and if you propose to violate that definition, you will have destroyed the very foundation of the theory.

Maybe what you really mean, is that the visible universe was in communication with entities outside of the visible universe. But under GR, if the communication was possible gravitationally, it was also by definition possible optically. IOW, we should be seeing the photons that arrived together with the gravity well.

My only thought is that the level 3 parallel universes (aka many worlds) are present, and the gravity is leaking from one (or more) of such universes to our, at that particular location. In one such universe there must something gone wrong. Possibly a super giant black hole is there, or the prevalent number of universes are having a major mass there. I guess that would also mean that the gravitons do exist. Also an additional non-empty space dimension is in the game, as an other explanation (but I can't really image how would that work in harmony with other phenomena). Anyway, I'm baffled (and also excited at the same time). I can't otherwise imagine, what "outside" of our observable universe could be doing this. Because when we're talking about an outside influencing an inside, we're talking a (further) past influencing a (closer) past.

in GR the lightspeed limit for gravity is tautological: it is so by definition,and if you propose to violate that definition,you will have destroyed the very foundation of the theory.

Not necessarily. The universal lightspeed limit of energy/change propagation is well consistent with what I was trying to propose. It should not be in conflict with GR as such. When matter is "constructed" from energy, it will be under the effects of the local gravity field instantly, because it did not just came to existence "from nothing". Only nothing can come from nothing, and nothing else.. (weird sentence, I know) :)

It originated from energy that was allready there - or in other words, "it jumped on an allready rolling wave" - and therefor there is no violation of GR involved in the whole process.

Gravity travels at the speed of light, so no gravitational effect from "outside our 'horizon'" can possibly have reached us yet

Maybe what you really mean, is that the visible universe was in communication with entities outside of the visible universe.

If the universe is infinite, then yes, it should very well be "in communication", or rather, in a "dynamic relation" with itself, at least to a certain extent.

But under GR, if the communication was possible gravitationally, it was also by definition possible optically. IOW, we should be seeing the photons that arrived together with the gravity well.

Yes, in fact, we should.. But all we "see" at the edge of our current observational capability, is just some strange noise, also called the CMB.

Maybe, just maybe - when considering a "local bang" model - the shockwave front of such blast should be quite dense (eg. high energy density) + travelling at relativistic speeds, and in this regard, it could possibly "smear"/deflect/deviate all the from outside incomming photons as it propagates, which would in turn limit our "optical" observation radius.

Bloodoflamb: I would like to remind you that GR was also "one of the most accurate descriptions of the fundamental nature of the universe ever proposed by humanity" when Einstein finished doctoring his equations to satisfy his view of the Universe, that it was static. He called this his "greatest blunder" when Edwin Hubble proved him wrong. You see, Hubble determined through observation that the Universe was not static, but actually that it was expanding.

I see a lot of disagreement about how gravity can have an effect on these galaxies. You have to realize that these galaxies can "see" further in the direction that they are oriented from us. So a galaxy that is 4 billion light years in the direction of some constellation can see 4 billion light years further in that direction than we can and we can see 4 billion light years in the exact opposite direction of the sky than that galaxy can see. As such, gravity (and light) past that galaxy can be seen by observers in that galaxy. Many people here are making the mistake of assuming that nothing lies beyond the observable universe as that implies we're at the center of the universe. This type of assumption has been proven wrong every time it's proposed. What makes more sense, that we're special and can see the entire universe or that we're not special and just can't see matter that's pulling on distant galaxies because it's beyond our visible distance?

What makes more sense, that we're special and can see the entire universe or that we're not special and just can't see matter that's pulling on distant galaxies because it's beyond our visible distance?

No, the question is, does the Universe, while under expansion, lose the ability to communicate from all points in space with itself due to expanding faster than light.

Can the Universe break it's own speed limit. Right now we think so, and if that's the case physics may not hold true at all points throughout. That's why these topics become so hotly debated.

" Maybe, just maybe - when considering a "local bang" model - the shockwave front of such blast should be quite dense (eg. high energy density) + travelling at relativistic speeds, and in this regard, it could possibly "smear"/deflect/deviate all the from outside incomming photons as it propagates, which would in turn limit our "optical" observation radius. "

PinkElephant: your statement that "gravity travels at the speed of light" is an assumption, purely theoretical. In fact, in my model of the Universe, gravity occurs as the interaction of objects in a dynamic web, or field, in which changes occur instantaneously, like entangled photons. That is why the effort to detect gravitational waves is an exercise in futility, likewise searching for the graviton. Experience over time will prove me right, just wait and see.

Most astrophysicists acknowledge that space is every bit as much a creation as is time and matter. Matter displaces space, yet the expanding Universe theory must account for the mutable nature of an expanding space in which the dynamic of the gravity field is constant and relative.

then light can only have traveled a maximum distance of 13.7 billion light-years in a straight line since the Big Bang.

13.7: I'm guessing as long as those nice polite photons travelling along the straight and narrow line didn't get subverted by any massive objects, dark matter and definitely did not get sucked into any dark flows since that seminal cosmic ejaculation. And wasn't that cosmos dark anyway for the first so many hundreds of thousands or even millions of years?

re: posts that observations of clusters could be influenced by objects beyond our observable universe (OU):

If there are any observations of influence on the clusters, those influences travel to us at the light speed (LS) with the observation of the influenced. This means the influence from the influencing object(s) (including visible light) would travel towards us and reach us at the same time as the influence on the clusters (or sooner). This means that the influencer would be in our OU. Or the reverse statement: If it's not in our OU, neither is an observable influence on any object in our universe.

Even if expansion between the influencer and the influenced is now greater than LS (having pushed the influencer beyond our OU since the influence), we still would be seeing the light from the influencer as it was when it influenced the cluster, even though at present day, the influencer is outside of our OU.

In both cases, we'd never see influences of any influencer outside our OU.

CSharpner: I don't think this is an accurate statement. It brings back the thought experiments where someone moving at .9 the speed of light turns on a flashlight. Then it seems as though the light from that flashlight is moving at 1.9 the speed of light. It is not.

Just because gravity is having an effect on a galaxy at the edge of OU at the speed of light doesn't mean that light coming from that galaxy is coming at us from 2X light speed.

typicalguy: Your statement is correct, but you misunderstood my statement. When I said "or sooner", I didn't mean the light from the influencer was faster than c. I meant that if the influencer is not exactly behind the influenced object from our line of site, then the light from the influencer travels directly to us which is a shorter distance than the influence from it to the influenced object, then the light from influenced object coming to us.

With three objects (us, the influenced, and the influencer) in a triangle, the straight line between us and the influencer is shorter than the angled line between the influencer, the influenced, and us. That's all. Just simple geometry.

Nothing goes faster than C. I'm just explaining that if we see the /influence/ on an object, then whatever's influencing it should also be in our visible universe. The light from all objects travels at C. The influence travels

Nothing goes faster than C. I'm just explaining that if we see the /influence/ on an object, then whatever's influencing it should also be in our visible universe. The light from all objects travels at C. The influence travels < = C.

But upon further reflection, there's one potential hitch: the "dark" gravitational influence might have occurred during the inflationary phase, or during the "dark ages", when no currently-visible light could propagate to us from the entity/entities exerting the influence. By the time the plasma filling the universe condensed enough to allow long-distance light transmission, the influencing objects might have been "inflated" beyond our light horizon. This, it seems to me, is one of only two conceivable scenarios that's consistent with GR. The other scenario, would be a "naked" embedded gravitational well in space, transient in nature, with no matter/energy to anchor it, resulting from a quantum fluctuation that got hugely magnified during the inflation.

If you travel 1 light year (any direction) away from Earth and put up your telescope, you can now see images of the universe 1 light year further out in that direction than your friends still on the Earth can. As such, that objects gravity is also effecting you and while the friend's on Earth cannot see or feel the object, they can see that object's gravitational effects on you. Because of the increasing rate of expansion of the Universe, you may never see or feel this light/gravity directly. That's what I think is going on.

As such, that objects gravity is also effecting you and while the friend's on Earth cannot see or feel the object, they can see that object's gravitational effects on you.

No they can't. They will only see the effects on you, when light emitted by you reaches them -- 1 year later, precisely at the same time that the gravitational effects you're experiencing reach them as well.

@PE,Not knowing what the consensus is, do we consider gravity as having a velocity equal to the speed of light, or is the phenomenon of gravity instantaneous over, and coterminate at every point throughout the universe, and only exerting greater or lesser force as a result of local differences in the distribution of mass?

In GR, gravity is limited to light-speed. Generally speaking, all forms of information transfer (communication) are light-speed limited. You can imagine communicating over long distances, by wiggling a mass, thus causing a slight ripple in gravitational field. If this wiggle could be perceived instantaneously everywhere, then you've just achieved instantaneous (i.e. in GR's framework, a-causal) communication. This would allow you to create all sorts of paradoxes within the framework of GR, by violating causality.

Since the entire mathematical edifice of GR is thus vitally dependent on light-speed propagation for gravity, one either accepts GR as empirically valid, or reverts back to the Newtonian view and all of its contradictions with observations.

Here's another way to see it: if the gravitational field gives you an instantaneous and absolute view of the universe, there's no longer any relativity of inertial observers, which invalidates both GR and SR.

@PE,Thanks for the gloss. I suppose GR will have to do, at least for now. There have always been aspects of it which seemed counterintuitive, at least for me. The abrupt truncation of my academic career didn't help, either. Thus my spotty understanding of some of these concepts, even though I find them endlessly fascinating.

Besides- you never know when some casual question may lead to a revolution in understanding.

As to the holographic model- the idea of all the information in the universe being contained inside a singularity, which then expands omnidirectionally, at the speed of light, and projecting itself into existence in it's own wake- well, that's causing some conceptual difficulties, too...for instance- with increasing distance, would not the amount of information eventually attenuate, as the holographic surface area increased?I'm starting to get a headache...

That's just speculative playing with numbers; no physics at all. The universe as an 3D sphere with surface area PI*r**2 (in natural units) is just an ad-hoc assumption.Reminds me of those number jugglers who see all cosmological and physical entities coded in the ancient pyramids.

Maybe I should have explained a bit more. Specifically, I like the association of gravity with entropy. This makes gravity an emergent phenomenon, rather than a fundamental force. It also keeps gravity local, like it is in GR. It helps explain why gravity is so much weaker, and perhaps altogether vanishing, at small scales, yet completely dominates at large scales over all other forces. And, as mentioned in the linked post, it even helps explain "dark energy" without introducing any exotic new entities. That's a lot of birds killed with one stone.

I've pondered something like this myself, though less formally. I've wondered if gravity were a "drag" on matter/energy, caused by some sort of congealing or interference in the spacetime medium from nearby matter/energy. The specifics of how something like this would work, depend highly on a detailed theory of the medium's fundamental structure. The entropic approach sidesteps the detailed theory, creating a more generic argument.

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